Methods and systems for analyzing particles and particularly sediments are well known in the art, as disclosed in U.S. Pat. Nos. 4,338,024 and 4,393,466, which are incorporated herein by reference. Such systems utilize a flow cell through which fluid samples (specimens) are passed, and a particle analyzer for capturing still frame images of the fluid passing through the flow cell. Thus, the flow cell positions and presents the sample fluid containing particles of interest for analysis. The more accurately that the sample fluid is positioned by flow cell, the better the analysis of the particles therein that can be made.
Typical flow cells cause the sample fluid, and a sheath fluid that buffers the sample fluid, to flow together from a large entry chamber into a small cross sectional examination area or region. The transition from the inlet or entry chambers to the examination region forms a hydrodynamic lens that squeezes both the sample fluid and the sheath fluid proportionally into the smaller space. Where the particles of interest are microscopic particles, the resulting cross-sectional space occupied by the sample fluid must be positioned within the depth of field of the analyzer, such as an optical system or a laser system, to obtain the best analytical information. For the best hydrodynamic focus, a large area of sheath flow must envelop the small area of sample fluid without any swirling or vortices. Thus, uniform flow of sample and sheath fluids through the flow cell is essential for optimal operation of particle analyzers.
Particle analyzer devices, such as those referenced above, often stain the sample fluid before it goes through the flow cell for image capture. The stain is a highly concentrated liquid, and it has been determined that the staining process requires a sample-to-stain ratio of approximately 166:1. Using a separate mixing chamber (e.g. with a spinning mixing bar) is not ideal because it delays the amount of time it takes to get the stained sample fluid to the flow cell, it increases the required amount of sample fluid, and it adds an additional component that can be difficult to wash or rinse between samples. In line mixers have also been used, such as helically coiled tubes, randomly-coiled tubes (also called “3D” and in some situations “knitted” tubes), zig-zag tubes with “sharp” corners, and ball-mixers (i.e. a string of small spheres in the flow stream). The more complex the in-line mixing path, the more efficient the mixing, but also the more difficult it is to manufacture and to clean between uses. Particle analyzer devices have a no-carryover requirement, where old samples must be thoroughly washed or rinsed away before a new sample is collected and analyzed. The more “corners” in a mixer, the more likely it is for the mixing device to accumulate debris (e.g. from old samples) and the more difficult it is to clean the mixer in-between samples. Some of these mixers require excessive amounts of the sample to efficiently operate. Lastly, some of these mixing devices introduce a significant pressure drop, which makes it difficult to provide the sample to the flow cell with the proper pressure and flow rate.
Reducing the amount of sample fluid that needs to be aspirated from the collection point can be accomplished by reducing the inner diameter (ID) of the delivery tubing used to carry the sample to the flow cell, and keeping any fluid mixer as small as possible. However, automated particle analyzers also need to detect whether or not fluid is flowing (i.e. is present) in the delivery tubing. A delivery tube with a 0.033 inch ID requires a satisfactory volume of sample, but the amount of fluid that represents presence of the liquid is only about 5 μL per centimeter of the tubing. One challenge then is performing fluid sensing in such small tubing, without requiring any additional tube length to provide for the fluid sensing, and without disturbing the flow of fluid that could adversely affect stain mixing and flow cell operation.
It is well known that the dielectric constant of water and solutions involving water is very high compared to that of air. Water below 55° C. has a dielectric constant from 70 to 88, while air is 1.00. For sensing the small amounts of water consistent with small tubes, capacitance-sensing electrodes external to the tube can be used. However, without extending the length of the tube, those electrodes will be quite small. With small electrodes (about 1 cm long), the capacitive difference between water in the tube and no water in the tube is typically less than 1 pF, which can be difficult to detect.
There are many capacitance sensors presently available that can work down to the picoFarad level, but the stability of their measuring process is not precise enough to allow unattended operation for very long before the measured threshold drifts beyond usable levels. To counteract this drift, some devices use a “continuous calibration” technique, but that presupposes an assumed format for the presence or absence of liquid in the tube. For example, Q-Prox (of Southampton, England) has a 6-channel capacitance-based push-button sensor IC chip. While this sensor apparently senses just a few pF of capacitance change, it requires an elaborate self-adjusting system of “calibrating” the sensor points so that the drift in the IC can be compensated away. This scheme assumes the absence of any user supplied liquid, and uses this assumption to allow its self-calibration. Such a scheme would not work well for a liquid sensing scheme where the presence of the liquid could be true or false without a known time-varying schedule.
There is a need for a fluid mixer that mixes with the smallest volume possible to minimize the amount of sample fluid that must be involved in the mixing process, where the mixing is performed in the shortest length possible to minimize the transit time along the mixing path. Such a mixer should not contain any places to “store contaminants, so that it is easy to clean after the sample fluid has passed through, and it needs to be easy to manufacture at a low cost. A sensitive but reliable liquid sensor is also needed, one that works with small diameter tubing and is compatible with the fluid sensor and flow cell operation.